pbl:
That is the case in the microlight accident we are working. There was a specific structural failure, and there was a loss of control and tumble. The question is which came first. Prosecution, following accident investigation authority, says failure caused tumble. We say equally (or more) likely that tumble caused failure. I was lucky to come across, and recommend the engagement of, a forensic structural engineer who has all but proved it by looking much more closely than others at some critical parts as well as at the design and certification basis of the aircraft.
Sounds like an interesting case. If you rule out fatigue failure based on metallurgical examination and can categorically say that no foreign objects were involved, then you're left with either under-designed structure (or structure which was not built to the design specs) or some event, such as a gust, which overloaded a correctly designed structure. This would be the case for failure causing tumble. But then, as you say, you could also argue tumble causing failure. Then I guess you have to look for the reason for the tumble. I look forward to reading about it when you publish. Did you work with BFU at all on this?